In the field of semiconductor device processing epitaxially deposited silicon is commonly used in a variety of applications. Basically, this deposition, also referred to as growth, involves the precipitation of silicon from a source gas onto a crystal lattice, such that the deposited silicon forms a structure which continues the crystal lattice. Conventionally used silicon-source gases include silane (SiH.sub.4), silicon tetrachloride (SiCl.sub.4), trichlorosilane (SiHCl.sub.3), and dichlorosilane (SiH.sub.2 Cl.sub.2), the details of typical processing are described in ADVANCES IN DICHLOROSILANE EPITAXIAL TECHNOLOGY, D. J. DeLong, Solid State Technology, Oct. 1972, pp. 29-34, 41, and in U.S. Pat. No. 3,945,864, METHOD OF GROWING THICK EPITAXIAL LAYERS OF SILICON, N. Goldsmith et al., issued Mar. 23, 1976. The quality and rate of silicon deposition is a strong function of such parameters as deposition temperature and specific gas composition, as elaborated upon in U.S. Pat. No. 3,239,372, METHOD OF PRODUCING SINGLE CRYSTALLINE SILICON, E. Sirtl, issued Mar.8, 1966, as well as in the previously cited references.
Epitaxial deposits of silicon have also been selectively grown within the apertures of a silicon dioxide (SiO.sub.2) mask on the surface of a monocrystalline silicon substrate. An example of such a process is described in SELECTIVE EPITAXIAL DEPOSITION OF SILICON, B. D. Joyce et al., Nature, Vol. 195, pp. 485, 6, Aug. 4, 1962. Selective epitaxial deposition has also been used to form a grid of monocrystalline silicon islands wherein the grid is specified by a particular center-to-center spacing of an array of apertures in a silicon dioxide layer, and wherein each silicon island overgrows the silicon dioxide surrounding each apertures a specific distance. An example of such an overgrown structure and its manufacturing method is described in THE "EPICON" ARRAY: A NEW SEMICONDUCTOR ARRAY-TYPE CAMERA TUBE STRUCTURE, W. E. Engeler et al., Applied Physics Letters, Vol. 16, No. 5, Mar. 1, 1970; THE EPICON CAMERA TUBE: AN EPITAXIAL DIODE ARRAY VIDICON, S. M. Blumenfeld et al., IEEE Trans., Vol. ED18, No. 11, Nov. 1971; and in U.S. Pat. No. 3,746,908, SOLID STATE LIGHT SENSITIVE STORAGE ARRAY, W. E. Engeler, issued July 17, 1973.
It has been recognized that while monocrystalline silicon will nucleate on a monocrystalline substrate, a monocrystalline layer will not be nucleated on an amorphous surface. That is, when a surface such as that of a silicon dioxide mask layer is subjected to an epitaxial deposition environment, a non-single-crystalline silicon film is typically deposited. The tendency of an epitaxial deposition process to form monocrystalline material on a monocrystalline nucleation site as opposed to non-monocrystalline material on adjacent mask areas is commonly referred to as the selectivity of the process. As disclosed in the literature, a host of interdependent factors influences this selectivity.
When silicon is epitaxially deposited, conventional deposition temperatures are in the range of approximately 900.degree. to 1300.degree. C. although as disclosed in U.S. Pat. No. 3,511,702, EPITAXIAL GROWTH PROCESS FROM AN ATMOSPHERE COMPOSED OF A HYDROGEN HALIDE, SEMICONDUCTOR HALIDE AND HYDROGEN, D. M. Jackson, Jr. et al., May 12, 1970, temperatures as low as 800.degree. C. and as high as 1400.degree. C. may be useable. In an effort to suppress the growth of non-monocrystalline silicon on the mask layer several approaches have been taken. In commonly assigned U.S. Pat. application Ser. No. 608,544, METHOD FOR GROWING MONOCRYSTALLINE SILICON THROUGH A MASK LAYER, J. F. Corboy, Jr. et al. filed May 10, 1984, now U.S. Pat. No. 4,578,142, issued Mar. 25, 1986 a two stage selective epitaxial deposition process is described. Basically, this process comprises providing a substrate having an apertured mask thereon and subjecting the substrate to a two stage deposition cycle. In the first stage silicon is deposited from a silicon-source gas. In the second stage, performed in situ, a portion of the deposited silicon is etched by subjecting the substrate to a silicon etching gas.
In EFFECT OF SUBSTRATE PREPARATION AND GROWTH AMBIENT ON SILICON SELECTIVE EPITAXY, H. M. Liaw et al., Proceedings of the Electrochemical Society, CVD IX, Cincinnati, Ohio 1984, pp. 463-475, it is indicated that using SiCl.sub.4 as a silicon-source gas yields better selectivity than using SiH.sub.2Cl.sub.2. In SELECTIVE EPITAXY USING SILANE AND GERMANE, D. J. Dumin, Vapour Growth and Epitaxy. J. Crystal Growth, Vol. 31, (1975), pp. 33-36, it is taught that selectivity is enhanced by using SiH.sub.4 as a silicon-source gas and growing at temperatures in the range of 1180.degree. to 1270.degree. C., whereas at low growth temperatures silicon is formed on top of the adjacent oxide mask. The interdependence of growth rate, selectivity, Si/SiO.sub.2 ratio (nucleation site area/mask area) and presence of HCl with the silicon-source gas during deposition is elaborated upon in SELECTIVE SILICON EPITAXY USING REDUCED PRESSURE TECHNIQUE, K. Tanno et al., Japanese Journal of Applied Physics, Vol. 21, No. 9, September 1982, pp. L564- L566. This reference indicates that by using a SiH.sub.2 Cl.sub.2 /HCl/H.sub.2 deposition system in the 900.degree.-1000.degree. C. range at a pressure less than 80 torr, a suitable HCl concentration for selective epitaxial growth is a function of the exposed Si/SiO.sub.2 surface area ratio.
In U.S. Pat. No. 4,497,683, PROCESS FOR PRODUCING DIELECTRICALLY ISOLATED SILICON DEVICES, G. K. Celler et al., Feb. 5, 1985, a selective epitaxial deposition process is described wherein an oxide mask is overgrown with non-monocrystalline silicon and subsequently converted to monocrystalline material by a heat treatment which uses the selectively deposited monocrystalline silicon as a nucleation seed. Further elaboration on the relationship between the Si/SiO.sub.2 area ratio and growth rate may be found in THE GROWTH AND ETCHING OF Si THROUGH WINDOWS IN SiO.sub.2, W. G. Oldham et al., J. Electrochem. Soc.: Solid State Science, Vol. 114, No. 4, April 1967, pp. 381-388.
In SELECTIVE ETCHING AND EPITAXIAL REFILLING OF SILICON WELLS IN THE SYSTEM SiH.sub.4 /HCl/H.sub.2, M. Druminski et al., Journal of Crystal Growth 31, (1975), pp. 312-316, wells are etched in a silicon surface and are subsequently refilled by selective epitaxy using an SiCl.sub.4 /H.sub.2 system at about 1200.degree. C. or an SiH.sub.4 /HCl/H.sub.2 system at about 1150.degree. C. This reference indicates that the flatness of the epitaxial deposit may be controlled by varying the HCl concentration during the deposition. In commonly assigned U.S. patent application Ser. No. 694,100, METHOD FOR FORMING UNIFORMLY THICK SELECTIVE EPITAXIAL SILICON, J. F. Corboy, Jr. et al., filed Jan. 23, 1985, it is disclosed that equally thick selective epitaxial deposits may be made within unequally sized apertures by appropriately varying the HCl flow rate during the deposition.
In SELECTIVE EPITAXY OF SILICON UNDER QUASI-EQUILIBRIUM CONDITIONS, E. Sirtl et al., Semiconductor Silicon, R. R. Haberrecht and E. L. Klein, Eds., New York: Electrochemical Society, May 1969, pp. 189-199, it is indicated that the selectivity of a silicon epitaxial deposition process can be made independent of the Si/SiO.sub.2 ratio so long as deposition occurs at quasi-equilibrium conditions in the Si/Cl(Br)/H system using SiCl.sub.4 as the source gas and a deposition temperature of approximately 1200.degree. C. However, it is indicated that despite the use of quasi-equilibrium deposition conditions, surface defects at the edges of the deposits always remain. Lastly, U.S. Pat. No. 3,661,636, PROCESS FOR FORMING UNIFORM AND SMOOTH SURFACES, J. M. Green, II, et al., May 9, 1972 describes the use of an SiCl.sub.4 /H.sub.2 deposition system at temperatures between approximately 1110.degree. and 1400.degree. C. so as to yield an epitaxial deposit that does not have ridges along the edge thereof, although the relationship between growth rate and the size of the deposit, i.e. the Si/SiO.sub.2 ratio is not addressed.
These references notwithstanding, we have recognized the desirability of performing a selective silicon epitaxial deposition which is independent of Si/SiO.sub.2 surface area ratio and nucleation site width through a wide range of HCl concentrations, and have discovered a means for achieving such a result.